I keep carbon inserts in case I have to remove medications at some point.
Carbon in filters are a relic from the no water change era of fishkeeping. Back then, tanks could smell very bad. People didn't like the swampy smell of a room with a tank. If you are into the online father fish group, you will probably know that smell, but thankfully that return to past mistakes hasn't really caught on. I can remember as a kid knowing if people had an aquarium as soon as I entered their home, which is impressive given how everything smelled of stale tobacco smoke back then.
The aquarium business needs to sell things to make money, and filters became too efficient at one point. You'd buy an AquaClear with its sponges and never need new media. And so, carbon inserts and cartridges that need to be replaced were invented - not for efficiency but for profitability. Carbon and purigen are in that class of goods.
So my answer? Any carbon is too much, unless you've had an ich outbreak and want to get the medicinal dyes out.